1. Field of the Invention.
This invention relates generally to cathode ray tube control systems and is concerned more particularly with a cathode ray type tube connected to control circuitry having means for varying cathode emission capability in accordance with adjustments in electron beam current level.
2. Discussion of the Prior Art.
A display tube of the cathode ray type generally comprises a tubular envelope having therein an electron gun disposed to direct an electron beam onto a spaced anode imaging screen adjacent a transparent faceplate of the tube. The electron gun usually includes a heated cathode aligned with a spaced control grid electrode and with further spaced beam-focusing electrodes. In operation, the cathode is heated to a predetermined temperature for emitting a copious supply of electrons which pass through the control grid electrode, in accordance with the biasing potential thereof, and are beamed onto a discrete area of the imaging screen. An electromagnetic yoke may be disposed about a portion of the envelope adjacent the exit end of the gun for establishing a variable magnetic field which deflects the electron beam over successive discrete areas of the imaging screen in scanning a well-known raster pattern thereon.
The imaging screen generally comprises a layer of phosphor material which fluoresces locally to produce a spot of visible light when a discrete region thereof is penetrated by electrons from the beam. Consequently, when scanned by the electron beam, the imaging screen produces a visible light display of the raster pattern, which is viewable through the adjacent faceplate of the tube. Each discrete portion of the display pattern has a brightness proportional to the value of electron current impinging on the associated discrete area of the imaging screen. Thus, by suitably modulating the current level of the electron beam during the scanning operation, informational data may be "written" in the display pattern produced by the imaging screen and viewed through the adjacent faceplate of the tube.
Generally, the steady-state value of electron beam current is controlled by adjusting the biasing potential of the control grid adjacent the cathode of the gun. Accordingly, a modulating signal voltage may be applied between the cathode and the control grid for altering the electron current of the beam in the desired manner for "writing" informational data in the display pattern. If the informational data cannot be easily read through the adjacent faceplate, the biasing potential of the control grid may be adjusted to increase the steady-state value of electron beam current passing through it from the cathode. Thus, the electron beam will produce on the imaging screen a display pattern which is brighter with respect to the ambient illumination environment and more readily viewable through the adjacent faceplate of the tube.
If the ambient illumination conditions change to a higher level, such as from moonlight to daylight, for example, the brightness of the display pattern on the imaging screen relative to the illumination environment will decrease. In order to restore the relative brightness of the display pattern, the biasing potential of the control grid may be adjusted to increase the steady-state value of electron current passing through it still further. Consequently, in operation, the cathode usually is maintained at a sufficiently high temperature for constantly supplying the copious electron emission required to obtain an adequately bright image in a specified maximum illumination environment. However, maintaining the cathode at relatively high operating temperatures increases the rate at which materials from the cathode coating and the base metal sublimate, thereby causing depletion of electron emission capability and development of other undesirable effects, such as grid emission and interelectrode leakage currents, for examples. Consequently, since the tube generally is not operating constantly under maximum illumination conditions, the electron emitting capacity of the cathode is depleted more rapidly than necessary; and the operational life of the tube is shortened considerably.
Therefore, it is advantageous and desirable to provide a cathode ray tube control system with a cathode ray type tube connected to control circuitry having means for preserving the operational life of the tube.